0..10 mV measurement with PIC

mesuty · Guest

Hi friends;

I must measure an 0..10mV DC signal with PIC micro. The system must have two set points and two relay outs for each. I am planning use 4x20 LCD display and 18F452 or similar device.

The most important trouble for me is input amplifier to PIC.

does anybody have an idea ?

Many thanks.

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

If 5V was your reference, you do get 4.88mV granularity in your readings. i haven't tried it before, but if you make, say 2.5V as your reference, you can get 2mv granularity in your readings.

If that is so, then you might not even need an op-amp.

Calamar · Posted: Tue Jul 12, 2005 11:59 am

You should use a simple non-inverter amplifier,
sure it�ll necesary a +/- power source, maybe
with a intrumentacion amplifier could you avoid it.
any rail-to rail coul do the same ?
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Daniel H. Sagarra
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kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

How much resolution do you need?

It was suggested that you add a non-inverting op-amp. That is your best bet, one with sufficient gain to map your 0 to 10mV to 0 to VREF.

Select an op-amp with rail-to-rail input and output. Go look at National Semiconductor, TI and Microchip (but I'm not sure Microchip as rail-to-rail Input AND Output models but it has been a while since I've looked).

Keep in mind too that your op-amp will add error and so select the type and "grade" appropriately.

TI has a nice program called Op-Amp Pro that will help you as well as a VERY good application note written by Ron Mancini called "Op-Amps for Everybody" (or something similar). National Semi's WebBench has been adding to their design tools and I believe they also have some good op-amp design tools. Both tools require very little gray matter on your part.
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Calamar · Posted: Tue Jul 12, 2005 2:14 pm

Yes Kender it doesn't requiere, but with 10mv I dont't now, the threshold for the most rail-to-rail op-amp is in the order of few mV, anyway for a conventional op-amp shouldn't be necessary if it keep away form ground (or Vss)
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Best Regards
Daniel H. Sagarra
La Plata (Argentina)

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

As already suggested I would use an OPAMP to boost the voltage. Things to watch for:

Currently I have been using the following OPAMPs quite a bit:

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

Kender,

I believe you're confusing the terms here:

Calamar · Posted: Wed Jul 13, 2005 8:46 am

Ttelmah · Guest

He did not mention the word 'rejection'.
There are two seperate figures. Common mode rejection, is the ability of an op-amp to reject signals where both inputs change together. Common mode input range, is the voltage range over which the inputs can operate. It is the latter that matters, in the context being talked about, and often does not include ground for single rail amplifiers. Seperately, there is also the issue, that the output impedance of many single rail amplifiers rises if the output is driven close to ground, which introduces errors in the ADC converter on the PIC, whic requires quite a low drive impedance to work properly. Adding an offset voltage may be worth considering if linearity right to ground is important.
Look at the Texas intruments INA122, for a reasonably priced single supply op-amp, with in input common mode range, that goes to 0.1v below the -ve supply rail (ground in single supply configurations).

Best Wishes